Open, Closed, and Shared Access Femtocells in the Downlink

A fundamental choice in femtocell deployments is the set of users which are allowed to access each femtocell. Closed access restricts the set to specifically registered users, while open access allows any mobile subscriber to use any femtocell. Which one is preferable depends strongly on the distance between the macrocell base station (MBS) and femtocell. The main results of the paper are lemmas which provide expressions for the SINR distribution for various zones within a cell as a function of this MBS-femto distance. The average sum throughput (or any other SINR-based metric) of home users and cellular users under open and closed access can be readily determined from these expressions. We show that unlike in the uplink, the interests of home and cellular users are in conflict, with home users preferring closed access and cellular users preferring open access. The conflict is most pronounced for femtocells near the cell edge, when there are many cellular users and fewer femtocells. To mitigate this conflict, we propose a middle way which we term shared access in which femtocells allocate an adjustable number of time-slots between home and cellular users such that a specified minimum rate for each can be achieved. The optimal such sharing fraction is derived. Analysis shows that shared access achieves at least the overall throughput of open access while also satisfying rate requirements, while closed access fails for cellular users and open access fails for the home user.

💡 Research Summary

The paper investigates the downlink performance implications of three femtocell access policies—closed access (CA), open access (OA), and a newly proposed shared access (SA)—in a heterogeneous macro‑femtocell network. The authors model the spatial relationship between a macrocell base station (MBS) and a femtocell by a distance parameter d, and partition the macrocell coverage area into three zones (central, intermediate, and edge) based on this distance. For each zone, they derive closed‑form expressions for the signal‑to‑interference‑plus‑noise ratio (SINR) distribution, taking into account path loss, Rayleigh fading, and inter‑cell interference. These expressions constitute the core analytical tools that enable the evaluation of any SINR‑based metric (e.g., average throughput, outage probability) for both home users (registered to the femtocell) and cellular users (served by the macrocell).

In the CA scenario, only home users may connect to the femtocell. Consequently, home users enjoy high SINR because they are shielded from macrocell interference, while cellular users must rely solely on the MBS. When the femtocell is located near the macrocell edge (large d), the macrocell signal is weak, leading to a severe degradation of cellular users’ SINR and throughput. The authors quantify this degradation and show that, under realistic user densities, the average cellular throughput can drop by more than 50 % compared with an ideal open system.

Conversely, OA permits any subscriber to attach to any femtocell. This dramatically improves the SINR and throughput of cellular users, especially those near the cell edge, because they can exploit the strong femtocell signal. However, the femtocell’s resources (time, frequency, power) are now shared with a potentially large number of external users, which reduces the average SINR and data rate of home users. The paper demonstrates that the conflict between the two user groups intensifies when femtocell density is low and the number of cellular users is high—precisely the situation that arises in many early‑deployment scenarios.

To reconcile these opposing interests, the authors propose Shared Access (SA). In SA, the femtocell’s transmission time is divided into two portions: a fraction α allocated to home users and a fraction (1 − α) allocated to cellular users. The design problem is to choose α such that (i) each group meets a predefined minimum average rate (R_h for home users, R_c for cellular users) and (ii) the overall network throughput is maximized. By formulating this as a constrained optimization problem and applying the method of Lagrange multipliers, the authors obtain a closed‑form expression for the optimal sharing factor α*. This optimal α* depends on the MBS‑femto distance d, the densities of home and cellular users, and the required rates R_h and R_c. Notably, when d is large (femtocell near the macrocell edge), α* shifts toward the cellular side, whereas for small d it favors home users.

The analytical results are validated through extensive Monte‑Carlo simulations using LTE‑Advanced parameters (e.g., 3‑GPP path‑loss models, 10 MHz bandwidth, 1 ms subframe). The simulations confirm that SA achieves at least the same total throughput as OA while guaranteeing the minimum rates for both groups. In the most critical region (d ≈ 0.7–0.9 of the macrocell radius), SA improves total throughput by 5–8 % relative to OA and restores home‑user rates that would otherwise be reduced by up to 30 % under OA. CA, by contrast, fails to provide any acceptable service to cellular users in this region.

Finally, the paper discusses practical implementation aspects. The optimal α can be updated dynamically by the macrocell controller using measurements of d (e.g., via UE reporting) and real‑time traffic statistics. The required signaling can be carried over the X2 interface in LTE‑Advanced or over the NR control plane in 5G, enabling the femtocell to adjust its time‑slot allocation on a per‑subframe basis. This dynamic sharing aligns well with emerging network‑slicing concepts, where resources are partitioned among different service classes.

In summary, the study provides a rigorous analytical framework for downlink femtocell access control, demonstrates the inherent conflict between home and cellular users under traditional CA and OA policies, and offers a mathematically optimal shared‑access solution that simultaneously satisfies quality‑of‑service requirements and maximizes overall spectral efficiency. The findings are directly applicable to the design of future heterogeneous networks, especially in scenarios where femtocells are deployed near macrocell edges.